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Subject: Re: RIP the [Fluxgate compass /benefits of 3 axis] ?
From: George Huxtable (george@XXX.XXX)
Date: Mon Feb 04 2002 - 09:22:39 EST
Jared Sherman said-
>George, with all this talk about the uncorrectable problems inherent in
>fluxgate compasses it seems that there is a point going unspoken.
>
>Perhaps if these errors are all so damaging, the fluxgate compass is an
>obsolete artifact that should be abandoned along with the sunstone and the
>astrolabe.
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Reply from George-
Well, let's keep a sense of proportion about all this.
What I was trying to get over to members is that there is nothing magic
about fluxgate instruments that makes them immune from the disturbances
that affect other magnetic sensors (needle compasses).
I didn't maintain that those disturbances were "uncorrectable" (I hope).
What I tried to point out was that they could be corrected, but it needed
several extra transducers to go with the fluxgate. Fast-responding sensors
of tilt-change, insensitive to linear accelerations, to determine the
short-term changes in the vessel's attitude, and slow-responding
gravity-sensors to establish the mean values of those tilts. Those sensors
do not need to be very expensive ones. A kit which combined a strap-down
3-axis fluxgate with those sensors could do a good job in measuring the
magnetic azimuth of the vessel's fore-and-aft line, with little
perturbation from the sea conditions.
Even without such electronic complications, however, a simple gimballed
fluxgate transducer does a reasonable job in steering most craft.
Performance gets worse as the sea gets rougher, as we all know. It gets to
the point where an experienced helmsman can do much better, partly because
he can see the plane of the horizon, but also because he can observe the
waves and steer the best path amongst them. Whereas even the best
self-steering system can only try to maintain, blindly, a constant magnetic
course.
But that experienced helmsman loses much of his advantage when it gets
dark. A single-hander has to sleep sometimes.
There is a requirement for improved magnetic steering systems, and the
technology to do it. My hope is that for self-steering, strap-down fluxgate
instruments, properly corrected for dynamic tilt changes, will come within
our reach before long.
But nothing is going to persuade me to leave my old Sestrel compass at home.
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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